There have been proposed a number of amorphous semiconductor films (including microcrystal semiconductor films) such as amorphous silicon film containing hydrogen atoms (H) or/and halogen atoms (X)(a-Si(H,X) film), amorphous silicon carbide film containing hydrogen atoms (H) or/and halogen atoms (X)(a-SiC(H,X) film), amorphous silicon nitride film containing hydrogen atoms (H) or/and halogen atoms (X)(a-SiN(H,X) film), amorphous silicon and germanium-containing film which contains hydrogen atoms (H) or/and halogen atoms (X)(a-SiGe(H,X) film) and amorphous germanium film containing hydrogen atoms (H) or/and halogen atoms (X) (a-Ge(H,X) film), which are usable as constituent element members in thin film transistors, electrophotographic photosensitive devices, image input line sensors, image pick-up devices, photovoltaic devices, solar cells, other various electronic devices and other optical devices. Some of these amorphous semiconductor films have been practically used.
For the formation of such deposited film, a radiofrequency glow discharge decomposition method (hereinafter referred to as "RF glow discharge decomposition method") has been generally employed.
However, there are disadvantages for the RF-glow discharge decomposition method such as the: decomposition rate is low; a film-forming raw material gas is likely to be polymerized in the vapor phase thereby causing powdery materials to form which are deposited on the circumferential wall of a deposition chamber; due to high gaseous pressure at the time of forming a deposited film a long period of time is required for the deposition chamber to be ready to continue the film-forming process; the efficiency of a film-forming raw material gas to be consumed for the formation of the deposited film is low, and because of this, the resulting product becomes unavoidably costly.
In order to overcome these disadvantages for the RF-glow discharge decomposition method, the public attention has been recently focused on a microwave plasma CVD method as an effective film-forming method which is free of the foregoing disadvantages which are found on the RF-glow discharge decomposition method. Various studies have been made on the microwave plasma CVD method.
For instance, an ECR microwave plasma CVD method and an apparatus for practicing said method are disclosed in Journal of Non-Crystalline Solids 77 and 78, pp. 813-816(1985) "high rate deposition of a-Si:H using electron cyclotron resonance plasma" by S. KATO and T. AOKI.
Japanese Patent Application Laid-Open Sho.60(1985)-186849 (based on U.S. patent application Ser. No. 580,086, filed Feb. 14, 1985) also discloses a microwave plasma CVD apparatus having an inner chamber circumscribed by a plurality of cylindrical substrates into which microwave energy from microwave introducing means is introduced to improve the utilization efficiency of a film-forming raw material gas.
For these proposed microwave plasma CVD apparatuses, there can be expected certain advantages in the utilization efficiency of a raw material gas and/or deposition rate in the casc of forming a deposited film. However, there are still problems for any of these apparatuses. That is, a film is often deposited and adhered onto a microwave introducing window connected to a waveguide or a microwave introducing antenna in the deposition chamber and this causes changes for microwave energy to be introduced thereinto with the progress of time. As a result, it becomes difficult to continuously form a desired deposited film. In order to eliminate these problems, it is required to periodically conduct repairing works on the foregoing microwave introducing means. In consequence, the resulting product becomes unavoidably costly.
Especially, for instance, in the case of preparing an electrophotographic photosensitive member using such conventional microwave CVD apparatus, it takes a long period of time to form its light receiving layer which is required to be as thick as 10 .mu.m or more. The deposition rate is relatively high and during the film-forming process, a substantial quantity of film is also deposited on the microwave introducing means.
Because of this, microwave energy which is introduced through the microwave introducing means into the deposition chamber during the film-forming process is likely to change with the progress of time. As a result, the light receiving layer exhibits such electric characteristics as variance in the layer thickness direction.
Another microwave plasma CVD method and an apparatus suitable for practicing said method, wherein plasma potential is controlled by the application of external bias voltage to form a deposited film, are disclosed in Japanese Patent Application Laid-Open Sho.61(1986)-283116 (based on U.S. patent application Ser. No. 734,576, filed May 15, 1985).
This microwave plasma CVD apparatus is of such constitution as shown in FIG. 4 (schematic perspective view).
In FIG. 4, numeral reference 401 stands for a substantially enclosed reaction chamber having a discharge space 406, with which a microwave transmissive window 402 made of a dielectric material such as quartz or alumina ceramics is provided. To the microwave transmissive window 402, a waveguide 403 extending from a microwave power source (not shown) is connected. The reaction chamber 401 is provided with an exhaust pipe 404 connected through a main valve (not shown) to an exhaust device (not shown). Numeral reference 407 stands for a holder for a substrate 405 to be positioned thereon, which is installed in the discharge space 406. The substrate holder 407 has an electric heater (not shown) to heat the substrate 405 to a predetermined temperature. The reaction chamber 401 is provided with a gas feed pipe 408 for supplying a raw material gas into the discharge space 406, which is extending from a reservoir (not shown) through mass flow controller (not shown). Numeral reference 410 stands for a bias electrode bar to apply a bias voltage into the discharge space 406, which is electrically connected to a bias power source 409. The formation of a deposited film using this microwave plasma CVD apparatus is carried out, for example, in the following way. That is, firstly, a proper substrate 405 is placed on the substrate holder 407 and the inside of the reaction chamber is evacuated by the action of the exhaust device to thereby bring the discharge space 406 to a vacuum of about 10.sup.-6 Torr Then, the electric heater is actuated to heat the substrate 405 to a predetermined temperature and it is kept at this temperature. Then, a film-forming raw material gas such as silane gas (SiH.sub.4) for example in the case of forming an amorphous silicon deposited film is introduced into the discharge space 406 through the gas feed pipe 408. Concurrently, the microwave power source is switched on to generate microwave of frequency in the range of 500 MHz or more (preferably, 2.45 GHz) whereby a microwave energy is introduced through the waveguide 403 and the microwave transmissive window 402 into the discharge space 406. At the same time, the bias power source 409 is switched on to thereby apply a bias voltage of DC, AC or RF into the discharge space 406 through the bias electrode bar 410. Thus, the film-forming raw material gas is excited and diassociated with the action of the microwave energy as applied while the potential of plasma generated being controlled with the action of the bias voltage as applied to cause the formation of a deposited film on the substrate 405.
In this case, it is possible to obtain a practically applicable deposited film of small square with certain expected effects due to the externally applied bias voltage.
However, there are various problems for the foregoing microwave plasma CVD apparatus in the case of forming a thick deposited film of large area for use. For example, in the preparation of an electrophotographic photosensitive device, the formation of the film requires the film-forming process to continue for a long period of time. Other problems are: (i) a film is thickly deposited not only on the microwave introducing means but also on the bias electrode to externally apply a bias voltage and because of this, the quantity of a microwave energy to be applied to plasma generated in the discharge space is unavoidably changed and at the same time, the bias voltage to be externally applied thereto is also unavoidably changed accordingly; (ii) as a result of the above occurrence (i), the resulting film develops characteristics of being undesirably uneven in the thickness direction; (iii) because of the above (i), it sometimes occurs such a happening that plasma becomes unstable causing interruption of discharge; and (iv) it is therefore extremely difficult to stably and repeatedly form a desirable thick deposited film. For the foregoing microwave plasma CVD apparatus, there are further problems in the case of forming a deposited film while continuously applying a bias voltage into the discharge space being maintained at a vacuum of 10.sup.-2 Torr or less for a long period of time that: (v) the quantitative ratios of electrons and ions present in plasma become high causing the plasma to be of extremely low resistance and because of this, there often occurs abnormal discharge such as arc discharge causing interruption of plasma discharge due to excessive electric current by the abnormal discharge; (vi) along with this, electric current excessively flows locally to the substrate and also in the space between the substrate and the bias electrode to result in damaging a film deposited on the substrate and also in removing the film deposited on the bias electrode which eventually contaminates into the film deposited on the substrate; (vii) in addition to the above (vi), electric current excessively flows also in the space between the bias electrode and the gas feed pipe and further in the space between the bias electrode and the microwave introducing means to result in damaging the apparatus; and (viii) this causes problems for the maintenance of the apparatus.
In view of the above, it is extremely difficult to stably mass-produce a desirable deposited film of large area using the forgoing microwave plasma CVD apparatus provided with means to externally apply a bias voltage into the discharge space.